Field of Invention
The present invention belongs to the field of vehicular wireless network communication, and relates to a transmission method of safety message, in particular to a method for transmitting safety message based on vehicular network.
Description of Related Arts
Vehicular network communication can improve the comfort level of vehicle driving, reduce the traffic accidents, and has already been an inevitable trend of modern automotive electronic technology development. Vehicular network communication is also a core technology of realizing Intelligent Transport System (ITS for short). A Wireless Access in Vehicular Environments (WAVE for short) system permits the vehicle driving on a frequency band of a Dedicated Short Range Communications (DSRC for short) technology. DSRC is a technology dedicated for vehicle communication internationally, IEEE 802.11p mainly prescribes the standards of a Media Access Control Layer (MAC for short) and a Physical layer (PHY for short) of a DSRC protocol, IEEE 1609 is a high-level standard using 802.11p as a communication protocol, with respect to a communication system architecture and a series of standardized services and interfaces defined when the wireless communication technology is applied to a vehicular environment.
DSRC/WAVE equipment works at a 5.9G frequency band, a working band occupies 75 MHz all together, wherein a guard band occupies 5 MHz, and the remaining 70 MHz is divided into seven 10 MHz channels, including one control channel (CCH for short) and six service channels (SCH for short). However, the time is divided into alternatively switched control channel intervals (CCHI for short) of 50 ms and service channel internals (SCHI for short) of 50 ms. For DSRC/WAVE sync interval as shown in FIG. 1, one CCH interval and one SCH interval (100 ms) form one sync interval, the CCH interval and the SCH interval are separated by guard intervals (GI) of 4 ms and are used for completing channel switching. The CCH channel transmits management message, for example WAVE service announcements and high-priority vehicle safety message, such as collision warning and emergency brake, SAE J2735 standard suggests that the transmission delay of the information of high priority is lower than 10 ms. In view of that information cannot be transmitted and received by single-radio equipment simultaneously in the same channel, WAVE is required to be in the CCH interval and all equipment must monitor the CCH in order to guarantee that all equipment can receive management message and safety message. Moreover, non-safety message is transmitted through the SCH interval. In order to transmit the non-safety message, in the SCH interval, the single-radio equipment has to switch to any one SCH channel from the CCH channels. However, the safety message to be transmitted only waits to be transmitted on the CCH channel when the next CCH interval comes. For the safety message transmission sensitive to delay, greater delay may be introduced in the scheme.
In the prior art, for example, a safety message broadcast mechanism in the vehicular environment is described in “Broadcasting message in multi-channel vehicular networks”, US 2011/0128902. It is assumed that information source vehicles can run on one or more channels in the patent. In the vehicular environment, once the vehicle detects that an accident happens in the SCH interval and a piece of safety message corresponding to the accident is generated, which indicates that the information source vehicles are going to broadcast “the current channel” and “the next channel” of the message. The current channel refers to the channel in which the vehicle detects the happening accident and transmits the safety message corresponding to the accident; however, the next (transmission) channel refers to (other) channels in which the source vehicles transmit the safety message afterwards. Neighbor node vehicles of the source vehicles become relay vehicles, and receive broadcast information transmitted from sources. Then, each relay vehicle receiving the safety message repeatedly broadcasts the information on other channels (channels other than “the current channel” and “the next channel”). The format of the transmitted safety message is as shown in FIG. 2. In order to guarantee that all vehicles running on any channels can receive the safety message in the SCH interval, that the same safety message must be repeatedly broadcast on all the channels in the patent certainly will cause great resource losses.
In real life, a scene that an automobile collision happens on a high-speed road is as shown in FIG. 3, for example, an automobile V1 collides with an automobile V2 at a certain moment, if the collision is successfully detected and the corresponding safety message is generated, theoretically, the safety message should be transmitted to surrounding vehicles instantly. However, if the moment of generating the message is at the end of the CCH interval by chance, the time required for transmitting the safety message exceeds the remaining CCH time, and then the message is stored and has an opportunity to be transmitted until the next CCH interval comes. Due to the existence of the SCHI and the guard intervals, the vehicles may return to the CCH interval for information transmission after waiting for 54 ms, while the delay may cause chain collision. Therefore, the delay of 54 ms is too long, and should be shortened.
Moreover, if the safety message is transmitted on the CCH channels in the SCH interval, the WAVE cannot guarantee that all the vehicles can monitor the safety message, since a single-radio vehicle has to switch to the SCH channels from the CCH channels for transmission if the non-safety message of the single-radio vehicle need to be transmitted in the SCH interval. This means that the single-radio vehicle cannot monitor the CCH channels and receive the safety message simultaneously in the SCH interval to cause serious consequences. In spite of this, if that the safety message is received and transmitted by the whole network in the SCH interval can be guaranteed, the transmission delay is greatly shortened, the transmission efficiency of the safety message is improved, and that the safety message is received and transmitted by the whole network in the SCH interval is a problem urgently needing to be solved.